Code - Effect
Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle - Alien Mode
Down, X, Left, Circle, Down, X, Left, Circle - Infinite Ammo
Left, Square, Up, Triangle, Right, Circle, Down, X - Invincibility
Left, Square, Right, Circle, Left, Square, Right, Circle - Invisibility
Right, Square, Up, Triangle, Right, Square, Up, Triangle - Slow Motion
Up, Triangle, Down, X, Up, Triangle, Down, X - Xen Gravity.
Uplink Mod:
For this code to work, you will need the demo disk from Playstation Magazine Issue 57 (The one with Jango Fett on the cover). From the Half-Life main menu go to Option and then the Cheats menu.
Enter the following cheat:
Triangle, Square, Left, Right, Circle, X, Up, Down.
It should open an extras menu on the options screen. Go to the extras menu and it should say "opening disc tray". When it opens remove the Half-Life disc and put in the issue 57 demo disc and it should load the Half-Life Uplink Mod.